Made importing LiteLLM optional

The app doesn't need litellm

src/game_reasoning_arena/backends/litellm_backend.py

@@ -3,7 +3,12 @@ LiteLLM backend for API-based inference.
 """
 
 import os
-import litellm
+try:
+    import litellm  # type: ignore
+except Exception as e:
+    raise ImportError(
+        "LiteLLM backend requested but 'litellm' package is not installed."
+    ) from e
 from typing import Any, Optional, Dict
 from .base_backend import BaseLLMBackend